MMO's have flirted with this idea for a long time, of making monsters a finite quantity. For example, silverscales are useful for their scale drops which can be crafted into armor. But they have a set breeding rate based on their total population. Once all the silverscales are killed, there's none left...but then you're left with a handful of power gamers who would literally go around wiping out species after species for the bragging rights and phat lewt. And tons of regular players who would throw a fit that content is being denied them. Asheron's Call used to have some world events and items that were quite literally unique, as in, there was ONE of them. In the whole game. Which got to be rather amusing on the PvP server, as getting the One Foozle Sword immediately marked you as a target for every living thing on the planet to come after.
It would be rather fascinating for a game to make monsters into a finite resource. Or even better, simulated a complete biosphere so that if you decimate the rat population in an area, maybe the wolf population drops, which hurts the wolf-eating Morgok population, forcing them to move or maybe start eating other things....the problem being that such a system would be crazy easy to throw out of whack through a little organized player action, and you have plenty of people who would wreck the world just for the lulz.
I could see DF eventually incorporating some of this though. It already tracks total world populations for most critters (and, as if to prove my point, some people have gone into Adventurer mode and quite deliberately depopulated the entire world of sentient life just to see if it can be done). An ideal MMORPG system would find a way to turn people like that into the object of a "Kill Big Bad Foozle" quest. Or somehow encourage conservation-minded players to organize in defense of nature.
I'm thinking of the infamous "Vorpal Bunny" in Asheron's Call. For those unfamiliar:
Asheron's Call had the unique trait of allowing monsters to gain XP from killing players. This would continue to accrue until the monster was slain and respawned. This meant that a particularly successful monster could lvl up beyond the "stock" version and gained appropriately in power. So somebody took a random lvl 1 brown rabbit a little ways outside one of the towns, and deliberately "fed" it kills by debuffing themself to a near-helpless state, then triggering aggro without damaging the rabbit (which had about 2hp). Over time, more people signed on to offer themselves as sacrifices, and another group formed to take turns guarding the rabbit and attempting to dissuade any other players from killing it. Eventually, they got it up over lvl 100, and it could quite easily kill an unprepared player in one shot. Eventually, another player group sought to kill it, and did so after an epic battle. (Turbine recognized the effort by adding an actual lvl-666 Vorpal Bunny NPC in a later patch).
So the idea of a self-organized player group trying to "Save the Mobs" isn't too far-fetched.